WBR0746

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Physiology
Sub Category	SubCategory::Musculoskeletal/Rheumatology
Prompt	[[Prompt::A researcher is studying the mechanisms of muscle contraction. In his experiment, he develops a new protein that has a high affinity to a special component in the muscle complex. Further evaluation of the new protein, he notes that it is capable of binding to a compound that normally covers myosin-binding sites on actin. The protein developed by the researcher most likely binds to which component of the muscle complex?]]
Answer A	AnswerA::Troponin
Answer A Explanation	AnswerAExp::Troponin is required to expose the myosin-binding sites on actin
Answer B	AnswerB::Tropomyosin
Answer B Explanation	AnswerBExp::Tropomyosin normally covers the myosin-binding sites on actin.
Answer C	AnswerC::Adenosine triphosphate (ATP)
Answer C Explanation	AnswerCExp::ATP is the form of energy required by the muscle to perform its cross-bridge cycling.
Answer D	AnswerD::Mitochondrion
Answer D Explanation	AnswerDExp::Mitochondrion is a cellular organelle needed to produce ATP. It is abundant in muscles, but has no direct role in the contraction process.
Answer E	AnswerE::Calcium (Ca)
Answer E Explanation	AnswerEExp::Ca is needed by troponin as a co-factor to expose the myosin-binding sites on actin.
Right Answer	RightAnswer::B
Explanation	[[Explanation::Contraction of muscle cells requires 2 co-factors: Adenosine triphosphate (ATP) and calcium (Ca). While ATP provides the energy necessary for the , Ca is required to block the binding of myosin to actin. Physiologically, trpononin, with the help of Ca, plays a role in exposing the actin-myosin binding sites by shifting the position of tropomyosin away from the myosin-binding sites on actin. As binding sites are uncovered, myosin may finally bind to actin. The process of "cross-bridge" cycling thus begins. Cycling releases energy from ATP in the form of adenosine diphosphate (ADP) and phosphate. In the absence of ATP, however, this energy-dependent cycling process may not occur. As such, the absence of ATP marks a phenomenon called "rigor mortis", where the muscles are continuously contracted due to the sustained actin-myosin binding. Educational Objective: Tropomyosin plays a role in covering the myosin-binding sites on actin. Reference: Krans JL. The sliding filament theory of muscle contraction. Nature Education. 2010; 3(9):66 Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::troponin, WBRKeyword::tropomyosin, WBRKeyword::calcium, WBRKeyword::ca, WBRKeyword::atp, WBRKeyword::adenosine, WBRKeyword::triphosphate, WBRKeyword::phosphate, WBRKeyword::rigor, WBRKeyword::mortis, WBRKeyword::contraction, WBRKeyword::complex, WBRKeyword::muscular, WBRKeyword::muscle, WBRKeyword::expose, WBRKeyword::uncover, WBRKeyword::cover, WBRKeyword::myosin, WBRKeyword::actin, WBRKeyword::binding, WBRKeyword::site, WBRKeyword::sites, WBRKeyword::cofactor, WBRKeyword::co-factor, WBRKeyword::cofactors, WBRKeyword::co-factors
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